A light line is an assembly of illuminating optical devices combined to project a uniform, thin line of light onto an object for line scan applications. A first type of light line device includes a plurality of linearly-arranged, separately illuminable light-emitting devices such as, by way of example, a one-dimensional, elongated array of light-emitting diodes (LEDs). A second type of light line includes a plurality of flexible optical fibers, each of which optical fibers includes opposed light-input and light-output ends. In order to form a light line, the light-output ends of the optical fibers are mechanically bound to one another to form a light-emitting face that is elongated along one dimension.
One advantage of a light line formed from flexible optical fibers is that the light-input ends of the fibers can be fed back to a single light source located remotely from the light-emitting light-line face, and more easily accessed when required. However, as with LED-based light lines, attendant to the use of optical fiber light lines is the issue of non-uniform illumination intensity at the light-emitting face. More specifically, disparate regions of equal area along the light-emitting face may illuminate with disparate intensities. There are two principal reasons for disparate illumination in an optical fiber light line. First, because the light-output ends of the constituent flexible fibers are simply grouped and bound to form the light-emitting face, any two regions of equal area along the face may include different numbers of fibers. Stated alternatively, the fiber-packing density may vary over the surface of the light-emitting face. Second, the constituent fibers may be disparately tilted relative to an illumination plane parallel to the light-emitting face. Even if every constituent fiber emits light over the same numerical aperture, a fiber whose light-emitting end is not orthogonal to the illumination plane will illuminate more dimly along an axis normal to the illumination plane than will a fiber whose light-emitting end is orthogonal to the illumination plane. Persons acquainted with the fabrication and use of light lines refer to regions of a light-emitting face that illuminate relatively brightly as “hot spots.” Hot spots are generally regarded as an undesirable attribute of optical fiber light lines. Accordingly, techniques have been developed for “calibrating” a light line. “Calibration” in this context refers to reducing the light-emission intensity of the most brightly illuminating fibers down to within an acceptable percentage difference in illumination of the most dimly illuminating constituent fibers.
One existing technique for reducing hot spots to improve light-emission uniformity over the face of a light line involves the mechanical scoring of bright regions. As part of this technique, an operator of a “calibration station” causes light to emit through the light-emitting face of the light line while capturing images of various regions of the light-emitting face with a camera linked to a data processing system. The captured images include “mapping data” representative of various regions of the actual light-emitting face and an indication as to relative illumination intensity. Using the captured images and associated region address and intensity information as a guide, the operator manually scores unacceptably bright regions with a scoring device (e.g., a scribe) in order to dim them. This technique has met with various difficulties. One problem is the scale on which the scoring must frequently be conducted; some of the bright regions are nearly or actually microscopic such that restricting the tip of the scribe to the confines of the bright region is extremely difficult at best. Accordingly, “collateral damage” to areas surrounding the bright region being scored is frequently encountered. Another difficulty is caused by inconsistent application of force to the scribe, such that bright regions are scored too deeply, thereby imparting irreparable damage to the light-emitting face. A still additional drawback of the current technique is that it requires the operator of the calibration station to manually move the camera with respect to the light line being inspected and calibrated in order to capture the various images. This manual movement is inefficient, and worse, introduces imprecision that may cause entire regions to go non-imaged and uncalibrated.
Accordingly, a need exists for an automated method of light-line calibration that reduces the inefficiencies and imprecision associated with manual calibration.